Piezoelectric bimorph type actuator

ABSTRACT

A piezoelectric bimorph type actuator which, given a constant applied voltage, is capable of greater displacement without suffering from the problem of generating a smaller force of displacement or increasing the actuator size. The piezoelectric bimorph type actuator which is cantilevered, includes an electrode plate, a pair of piezoelectric plates provided on both sides of the electrode plate, and a voltage source for applying a voltage between each piezoelectric plate and the electrode plate so as to displace a free end thereof, wherein a ratio t 2  /t 1  between the thickness t 2  of the electrode plate and the thickness t 1  of each piezoelectric plate is set to be 0.42-1.11 times as much as a ratio E 2  /E 1  between the Young&#39;s modulus E 2  of the electrode plate and the Young&#39;s modulus E 1  of each piezoelectric plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piezoelectric bimorph type actuator,in particular, a piezoelectric bimorph type actuator suitable for use inthe shutter mechanism of a camera.

2. Description of the Related Art

Piezoelectric bimorph type actuators of the type contemplated by thepresent invention have a piezoelectric plate provided on both sides ofan electrode plate. In a typical case, one end of the actuator iscantilevered while the other end thereof is permitted to move freely.With this arrangement, a voltage is applied between each piezoelectricplate and the electrode plate so as to displace the free end. Thisdisplacement is typically used to drive a mechanism such as the shuttermechanism of a camera.

The displacement of conventional actuators can be increased by applyingan increased voltage. One may readily figure out that in order toprovide a sufficient stroke to drive the shutter mechanism of a cameraunder a constant voltage, it is necessary to extend the free end orreduce its thickness. However, the result of this approach is that theforce (of displacement) generated at the free end is insufficient todrive the shutter mechanism.

Thus, in order to produce the necessary stroke while generating adesired force of displacement, the above-described approach may becombined with increasing the dimension of the actuator in the directionof width. In this case, however, the actuator size becomes bulky,potentially causing inconveniences when incorporating it into the camerawhich is a small device.

A further problem with the conventional approach of increasing thedisplacement of an actuator by extending its free end or reducing itsthickness is that the resonant frequency (natural frequency) of theactuator decreases resulting in potential deterioration in itsoperation.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these circumstancesand has as an object to provide a piezoelectric bimorph type actuatorwhich, given a constant applied voltage, is capable of greaterdisplacement without suffering from the problems of generating a smallerforce of displacement or increasing the actuator size.

The above and other objects of the present invention can be achieved bythe provision of a piezoelectric bimorph type actuator which iscantilevered, including an electrode plate, a pair of piezoelectricplates provided on both sides of the electrode plate, and means forapplying a voltage between each piezoelectric plate and said electrodeplate so as to displace a free end thereof, wherein the Young's modulusE₂ of the electrode plate is larger than the Young's modulus E₁ of eachpiezoelectric plate.

Further, the objects of the invention can be achieved by the provision apiezoelectric bimorph type actuator which is cantilevered, including anelectrode plate, a pair of piezoelectric plates provided on both sidesof the electrode plate, and means for applying a voltage between eachpiezoelectric plate and the electrode plate so as to displace a free endthereof, wherein a ratio t₂ /t₁ between the thickness t₂ of theelectrode plate and the thickness t₁ of each piezoelectric plate is setto be 0.42-1.11 times, preferably 0.5-1.1 times as much as a ratio E₂/E₁ between the Young's modulus E₂ of the electrode plate and theYoung's modulus E₁ of each piezoelectric plate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below serveto explain the principles of the invention. In the accompanyingdrawings:

FIG. 1 is a side view showing schematically the construction of apiezoelectric bimorph type actuator according to an embodiment of thepresent invention;

FIG. 2 is a graphical representation showing the relationships between athickness ratio of a piezoelectric bimorph type actuator and each ofthree parameters that are related to the change in an amount ofdisplacement δ; and

FIG. 3 is also a graphical representation showing the relationshipsbetween the thickness ratio of the piezoelectric bimorph type actuatoraccording to the embodiment and each of the amount of displacement and aforce caused by the displacement occurring in that actuator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail with reference to theaccompanying drawings. FIG. 1 is a side view of a piezoelectric bimorphactuator according to that embodiment. The piezoelectric bimorph typeactuator generally designated by 1 comprises an electrode plate 2 madeof an alloy that contains 42 percent nickel alloy, and a pair ofpiezoelectric plates 3 made of lead titanate zirconate (PZT) ceramicmaterial. The piezoelectric plates 3 have the same thickness and arecemented to opposite sides of the electrode plate 2. The size of theactuator 1 is such that the effective length L is 38 mm, the totalthickness t is 550 μm and the width is 5.5 mm.

The thickness ratio of the actuator 1 as expressed by t₂ /t₁, or theratio between the thickness t₁ of each piezoelectric plate 3 and thethickness t₂ of the electrode plate t₂ 2, is set as follows in terms ofthe ratio between the Young's moduli of the two plates, E₂ /E₁.

The Young's modulus E₁ of piezoelectric plate 3 is 6.7×10¹⁰ N/m² whereasthe Young's modulus E₂ of electrode plate 2 made of an alloy thatcontains 42 percent nickel is 14.5×10¹⁰ N/m² ; hence, the ratio betweenthe two values, E₂ /E₁, is 2.16. The thickness ratio t₂ /t₁ of the twoplates is set to be 0.42-1.11 times as much as the value 2.16 of E₂ /E₁,namely, at 0.9-2.4. Stated specifically, the thickness t₁ of eachpiezoelectric plate 3 is in the range of 125.0-189.7 μm whereas thethickness t₂ of the electrode plate 2 is in the range of 170.6-300.0 μmin association with the corresponding value of t₁. If the thicknessratio t₂ /t₁ is set at values that satisfy the above-specifiedrelationship with the Young's modulus ratio E₂ /E₁, various versions ofpiezoelectric bimorph type actuator 1 that are made of the same materialand which have identical appearance and shape will assume a maximumvalue for each of the amount of displacement and the force ofdisplacement that is generated. The reasons are stated below.

The amount of displacement of a piezoelectric bimorph type actuator isgenerally expressed by the following equation:

    δ=NVL.sup.2 /2K                                      (1)

where δ: the amount of displacement

N: N=d₃₁ E₁ W(t₁ +t₂)/2

where d₃₁ : piezoelectric constant,

E₁ : the Young's modulus of each piezoelectric plate

W: the width of each piezoelectric plate,

t₁ : the thickness of each piezoelectric plate,

and

t₂ : the thickness of the electrode plate.

V: applied voltage;

L: the effective length of the piezoelectric bimorph type actuator; and

K: the bending stiffness of the actuator.

The equation (1) is rewritten as:

    δ=A(N/K)                                             (2)

where A: VL² /2

If A in equation (2) is a constant (i.e., the values of both L and V areconstant), the amount of displacement δ will vary with the value of N/K.

If, with the overall size of the piezoelectric bimorph type actuatorbeing held constant, the thickness ratio t₂ /t₁ between the electrodeplate 2 and each piezoelectric plate 3 is varied, the values of N and Kwill change. Particularly in the case where the Young's modulus E₂ ofthe electrode plate 2 is greater than the Young's modulus E₁ ofpiezoelectric plate 3, the rate of increase in the value of K willexceed the rate of increase in the value of N at a certain point asshown in FIG. 2, so the value of N/K will have a maxima (in FIG. 2, thevertical axis represents the variations in the values of K, N and N/K inresponse to the change in the thickness ratio t₂ /t₁). Since N/K has amaximum value, equations (1) and (2) indicate that the amount ofdisplacement δ also has a maximum value. In other words, given theoverall size of actuator, the ratio t₂ /t₁ between the thicknesses ofelectrode plate 2 and piezoelectric plate 3 at which a maximum amount ofdisplacement δ is obtained assumes a constant value.

The value of K also varies with the values of Young's moduli, E₁ and E₂,of piezoelectric plate 3 and electrode plate 2 and, thus, the degree ofchange in the value of K as seen in FIG. 2 changes with the ratio ofYoung's modulus E₂ /E₁. Therefore, the thickness ratio t₂ /t₁ at which amaximum value of displacement δ, namely, a maxima of the value of N/K,is obtained will be determined by the ratio of Young's modulus E₂ /E₁.

Based on these observations, the present inventors prepared varioussamples of a piezoelectric bimorph type actuator of the same size as theactuator 1 under discussion using the same material but with differentvalues of thickness ratio t₂ /t₁ and then measured the amount ofdisplacement that occurs upon application of a constant voltage. Theresults are shown in FIG. 3. The measurements of δ were conducted with adrive circuit being operated in such a way that a voltage (V) of 160volts is applied to the actuator 1 via switch S as shown in FIG. 1. Thecurve indicated by a solid line in FIG. 3 represents the variation ofdisplacement δ in response to the change in the thickness ratio t₂ /t₁.

All samples of piezoelectric bimorph type actuator under measurement arecompletely identical to the version designed in accordance with theembodiment. It is composed of two PZT piezoelectric plates cemented toan electrode plate made of an alloy that contains 42 percent nickel,with the size being such that the effective length (L) is 38 mm, thewidth is 5.5 mm and the overall thickness (t) is 550 μm. As alreadymentioned in connection with the embodiment, E₂ /E₁ or the ratio betweenthe Young's moduli of an alloy that contains 42 percent nickel and PZTis 2.16.

As is clear from FIG. 3, the amount of displacement δ has a maximumvalue and at values in the vicinity of the maxima, δ changes following acomparatively flat and smooth curve. If it is assumed that the effectiverange, H, of displacement δ extends from 95% to 100% of the maxima, thethickness ratio t₂ /t₁ at which the amount of displacement δ is withinthe effective range H will lie between 0.9 and 2.4. Since the ratio E₂/E₁ of Young's modulus for each of the actuator samples undermeasurement, is 2.16, the following relationship holds good between thethickness ratio for producing the effective amount of displacement δ(t₂/t₁ =0.9-2.4) and the ratio of Young's modulus E₂ /E₁ (=2.16):

0.42×(E₂ /E₁)≦t₂ /t₁ producing the effective amount ofdisplacement≦1.11×(E₂ /E₁).

To put it simply, this relation states that the useful (maximum) amountof displacement δ is attained if the thickness ratio t₂ /t₁ is includedwithin the range over which it is 0.42-1.11 times as much as the ratioof Young's modulus E₂ /E₁. In the light of this observation, thepiezoelectric plates 3 and electrode plate 2 in the actuator 1 of theembodiment under consideration have thicknesses of the values alreadyspecified above. Stated more specifically, the thickness t₁ of eachpiezoelectric plate is within the range of 125.0-189.7 μm whereas thethickness t₂ of the electrode plate is calculated as 170.6-300.0 μm bysubtracting (2×t₁) from the overall thickness. By adjusting thethickness ratio t₂ /t₁ to assume values within the range determined bythose values, the amount of displacement δ will assume a substantiallymaximum value.

When performing the experiment for measuring the amount of displacementin the manner described above, the present inventors also measured theforce of displacement generated and the results are plotted in FIG. 3 bythe curve indicated by a dashed line. As the curve shows, the generatedforce of displacement has a maximum value as the amount of displacementδ and the position of that maxima is substantially the same as that ofthe amount of displacement δ. One can, therefore, conclude that if thethickness ratio t₂ /t₁ is set to lie within the range of 0.42-1.11times, preferably 0.5-1.1 times as much as the ratio E₂ /E₁ (the ratioof Young's modulus) as in the present invention, a substantially maximumvalue can be attained not only for the amount of displacement δ but alsofor the generated force of displacement.

As described on the foregoing pages, the piezoelectric bimorph typeactuator of the present invention has the advantage that, given aconstant applied voltage, it is capable of producing a greater amount ofdisplacement without requiring a change in shape or causing a drop inthe generated force of displacement. This has made it possible to meetthe requirements for improved operating characteristics and smallersize, which have heretofore been difficult to satisfy at the same time.

In addition, the operating characteristics of the piezoelectric bimorphtype actuator can be improved without modifying the shape parameterssuch as length and thickness of the device taken as a whole, and hencethere is no chance of its resonant frequency becoming low enough tocause deterioration in response characteristics, with the resultingimprovement in the characteristics of control over driving on aclosed-loop system.

What is claimed is:
 1. A piezoelectric bimorph type actuator which iscantilevered, comprising an electrode plate, a pair of piezoelectricplates provided on both sides of said electrode plate, and means forapplying a voltage between each piezoelectric plate and said electrodeplate so as to displace a free end thereof, wherein a ratio t₂ /t₁between the thickness t₂ of said electrode plate and the thickness t₁ ofeach piezoelectric plate is set to be 0.42-1.11 times as much as a ratioE₂ /E₁ between the Young's modulus E₂ of said electrode plate and theYoung's modulus E₁ of each piezoelectric plate.
 2. The piezoelectricbimorph type actuator as defined in claim 1 wherein the ratio E₂ /E₁ issubstantially 0.9-2.4.
 3. The piezoelectric bimorph type actuator asdefined in claim 1 wherein the thickness t₁ of said each piezoelectricplate is substantially 125.0-189.7 μm and the thickness t₂ of saidelectrode plate is substantially 170.6-300.0 μm.
 4. The piezoelectricbimorph type actuator as defined in claim 1 wherein said eachpiezoelectric plate is made of lead titanate zirconate (PZT) basedceramic material and said electrode plate is made of an alloy thatcontains 42 percent nickel.
 5. A piezoelectric bimorph type actuatorwhich is cantilevered, comprising an electrode plate, a pair ofpiezoelectric plates provided on both sides of said electrode plate, andmeans for applying a voltage between each piezoelectric plate and saidelectrode plate so as to displace a free end thereof, wherein theYoung's modulus E₂ of said electrode plate is larger than the Young'smodulus E₁ of each piezoelectric plate and a ratio t₂ /t₁ between athickness t₂ of said electrode plate and a thickness t₁ of eachpiezoelectric plate is set to be 0.42-1.11 times as much as a ratio E₂/E₁ between the Young's modulus E₂ of said electrode plate and theYoung's modulus E₁ of each piezoelectric plate.
 6. The piezoelectricbimorph type actuator as defined in claim 5 wherein the ratio E₂ /E₁ issubstantially 0.9-2.4.
 7. The piezoelectric bimorph type actuator asdefined in claim 5 wherein the thickness t₁ of said each piezoelectricplate is substantially 125.0-189.7 μm and the thickness t₂ of saidelectrode plate is substantially 170.6-300.0 μm.
 8. The piezoelectricbimorph type actuator as defined in claim 5 wherein said eachpiezoelectric plate is made of lead titanate zirconate (PZT) ceramicmaterial and said electrode plate is made of an alloy that contains 42percent nickel.
 9. A piezoelectric bimorph type actuator which iscantilevered, comprising an electrode plate, a pair of piezoelectricplates provided on both sides of said electrode plate, and means forapplying a voltage between each piezoelectric plate and said electrodeplate so as to displace a free end thereof, wherein the Young's modulusE₂ of said electrode plate is larger than the Young's modulus E₁ of eachpiezoelectric plate and a ratio t₂ /t₁ between a thickness t₂ of saidelectrode plate and a thickness t₁ of each piezoelectric plate is set tobe 0.42-1.11 times as much as the ratio E₂ /E₁.
 10. The piezoelectricbimorph type actuator as defined in claim 9 wherein said electrode plateis made of metal.
 11. The piezoelectric bimorph type actuator as definedin claim 9 wherein the ratio E₂ /E₁ is substantially 0.9-2.4.
 12. Thepiezoelectric bimorph type actuator as defined in claim 9 wherein thethickness t₁ of said each piezoelectric plate is substantially125.0-189.7 μm and the thickness t₂ of said electrode plate issubstantially 170.6-300.0 μm.
 13. The piezoelectric bimorph typeactuator as defined in claim 9 wherein said each piezoelectric plate ismade of lead titanate zirconate (PZT) based ceramic material and saidelectrode plate is made of an alloy that contains 42 percent nickel.